Nuclear reactors have coolant circulating at high pressures through a coolant circuit that flows past a reactor vessel which heats the coolant producing steam that turns a steam generator to produce energy. Corrosion and wear in a cooling system produces impurities in the coolant. These unwanted impurities must be removed to sustain efficient energy production and transfer of heat. It is known to continuously remove or bleed coolant from a primary coolant circuit to a secondary circuit that removes unwanted impurities. This bleeding is termed letdown flow. Typically, coolant in the secondary circuit is communicated to a letdown cooler before passing to a demineralizer where impurities are removed. Next, the coolant is stored in a makeup surge tank until being pumped back into the primary coolant circuit by a constant speed pump. Flow control valves located downstream of the constant speed pump, are provided to regulate the rate of flow and pressure of the coolant as it is being pumped back into the primary circuit. This replenishing is called charging flow.
Loss of coolant is known to occur due to leaks, damaged seals, or during the purification process. Therefore, the makeup surge tank needs to have sufficient storage capacity to enable replacing lost coolant and accommodate for changes in the total coolant volume in the primary circuit. Varying the volume of coolant in the primary circuit also controls the concentration of boron in the primary circuit and is critical for the efficient operation of a nuclear reactor plant.
Using control valves to determine the rate of charging flow and letdown flow has many drawbacks. For example, control valves are subject to wear and require maintenance. In addition, maintenance personnel could be exposed to radioactive materials when servicing worn out control valves. Further, a constant speed pump develops constant high pressures which must be reduced by throttling at the control valves to achieve a desired lower pressure. Thus, energy is wasted because pump energy is added to pressurize the coolant to a constant pressure, which energy is in turn dissipated during throttling to a lower pressure before re-introducing coolant into the primary circuit.
Charging flow and letdown flow are varied to change the concentration of boron in the coolant for reactivity control or to compensate for volume changes caused by temperature changes in the reactor coolant system. However, power losses are associated with varying the makeup and letdown flows. Therefore, there is an operating incentive to operate at minimum letdown and makeup flows that still provide a proper coolant chemistry. In a conventional system, operating at minimum letdown and makeup flows requires maximum throttling by the control valves. Maximum throttling, in turn, involves maximum energy being wasted.